An envelope tracking (ET) system is known as a technology for improving the electrical power efficiency of amplifiers used in transmission devices, such as mobile terminals or radio relay devices. The envelope tracking system is a technology for controlling a drain voltage in accordance with the variation in the envelope curve of a transmission signal (hereinafter, referred to as an “envelope”). The envelope tracking system can allow amplifiers to be operated in a state close to saturation, and thus it is possible for the amplifiers to be operated with high efficiency.
However, in the envelope tracking system, because drain voltage varies, the output impedance of an amplifier is inconsistent with the impedance of the output load, thus causing the power efficiency of the amplifier to be reduced. In particular, if the amplifier is operated at a low power output, the power efficiency of the amplifier drops to the lowest efficiency.
FIG. 8 is a schematic diagram illustrating a power efficiency curve of a commonly used amplifier that uses an envelope tracking system. The horizontal axis in FIG. 8 indicates output power [dBm] and the vertical axis in FIG. 8 indicates power efficiency [%]. As indicated by a curve 300 illustrated in FIG. 8, the power efficiency of the amplifier that uses the envelope tracking system is reduced as the output power is reduced in accordance with the drop in the drain voltage. Specifically, if the amplifier is operated at a low power output, the power efficiency of the amplifier drops to the lowest efficiency.
Accordingly, various kinds of amplifiers have been proposed whose power efficiency obtained when the amplifiers are operated at a low power output is improved. For example, with conventional amplifiers, two amplifying elements having different operating power are connected in parallel and these two amplifying elements are selectively operated in accordance with the voltage of an envelope of a transmission signal.
Specifically, a conventional amplifier detects the envelope of the transmission signal. Then, the amplifier compares the voltage of the envelope with a predetermined reference voltage. Thereafter, in accordance with the comparison result of the voltage of the envelope and the predetermined reference voltage, the amplifier selects, from the two amplifying elements, an amplifying element that amplifies the transmission signal. Then, the amplifier controls, in accordance with the envelope, the drain voltage of the selected amplifying element. Accordingly, if the voltage of the envelope is equal to or greater than the reference voltage, the amplifier can allow the amplifying element whose operating power is relatively high to be operated, whereas, if the voltage of the envelope is lower than the reference voltage, the amplifier can allow the amplifying element whose amplifying element is relatively low to be operated. Consequently, it is possible to improve the power efficiency of the amplifier when it is operated at a low power output.
FIG. 9 is a schematic diagram illustrating a power efficiency curve of a conventional amplifier that selectively operates two amplifying elements in accordance with the voltage of the envelope of the transmission signal. The horizontal axis in FIG. 9 indicates output power [dBm] and the vertical axis in FIG. 9 indicates power efficiency [%]. A straight line 301 indicated by the broken line illustrated in FIG. 9 indicates the output power obtained when the voltage of the envelope becomes the predetermined reference voltage. As indicated by a curve 302 illustrated in FIG. 9, if the voltage of the envelope is equal to or greater than the reference voltage, the amplifying element whose operating power is relatively high is operated; therefore, the power efficiency is kept in a high state. In contrast, as indicated by a curve 303 illustrated in FIG. 9, if the voltage of the envelope is lower than the reference voltage, the amplifying element whose operating power is relatively low is operated; therefore, the power efficiency is also kept in a high state.
Patent Document 1: Japanese Laid-open Patent Publication No. 2002-374128
Patent Document 2: Japanese Laid-open Patent Publication No. 62-277806
Patent Document 3: Japanese Laid-open Patent Publication No. 2009-10484
However, with the conventional technology, there is a problem in that the power efficiency is degraded due to the variation in the environment in which the amplifier is being used, such as a temperature change or a variation in accuracy of parts.
For example, there may be a case in which, due to a temperature change, an excessive drain current is output from a power supply that supplies the drain voltage and thus the efficiency curve of the amplifier may be shifted. With the conventional technology described above, even if the efficiency curve of the amplifier is shifted, the reference voltage that is used to compare the voltage of the envelope is still fixed; therefore, the power efficiency may possibly significantly drop. It is assumed, for example, that the power efficiency of the amplifying element indicated by the curve 302 illustrated in FIG. 9 is shifted to a curve 302a illustrated in FIG. 9 due to a temperature change. In such a case, the power efficiency with respect to the output power when the voltage of the envelope becomes the predetermined reference voltage indicated by the straight line 301 significantly drops, as indicated by an arrow 304. Accordingly, with the conventional technology described above, the power efficiency may possibly drop due to the variation in the environment in which the amplifier is being used, such as a temperature change or a variation in the accuracy of parts.